The term "tapping" is used to mean making an access point through the side of the tubing via a short, blind, tail-pipe (which may be of circular or rectangular section) of much smaller diameter and oriented perpendicularly to the axis of the tubing. The access point through the side is used when working on the tubing. The tapping point is opened by taking a cover off the tail-pipe, thus enabling all sorts of devices to be passed into the tubing for maintenance and repairs (in particular, flexible baloon plugs may be inserted and inflated inside the tubing to seal the circuit from the outside world) or for inspecting tubing integrity (in particular by gamma radiography).
Up to now, such tapping points have been provided at appropriate locations at the design stage, and they have been made during initial construction of the installation. Providing an additional tapping point in pre-existing tubing has required major work of long duration in which the portion of the installation to be modified is isolated, emptied, cooled, and disassembled.
Preferred implementations of the invention enable such tapping points to be made without requiring the installation to be closed down for a long period of time, in particular by keeping the circuit under a controlled atmosphere of inert gas (eg. argon) before, during, and after the operation, and by working on the tubing while it is still hot, ie. at temperatures of up to 180.degree. C.
These properties make it possible, in particular, to take action quickly on the sodium circuits in a nuclear power station, which action used to be impossible, once the installation had been put into operation, and without a major undertaking.